Rail cars typically consist of a car body, where cargo is loaded, and a pair of trucks, which are wheel and suspension assemblies upon which the car rests. The truck is configured to match the rails upon which the rail car moves. The connection between the car body and the trucks enables the car body to roll relative to the trucks and allows the truck to swivel, or yaw, relative to the car body. The connection may include centerplates or bearings and a pair of pins positioned on opposite ends of the bottom of the car. The pins and centerplates are configured to enter corresponding openings in the trucks. The car body-truck connection is intended to permit passive steering of the wheelsets and allow the trucks and car to follow the track without derailing. One problem with this connection configuration is that it may lead to a phenomenon known as “hunting,” which is a resonant condition exhibited by the car in response to the forces from the wheel/rail interaction at certain speeds. This resonance condition is evidenced by greatly increased lateral and yaw motions of the truck and car body, leading to increased wear of truck and track components. The resulting increased movement also increases stress on the rails.
In an effort to reduce hunting, (resonance) and thereby stress on the rails, constant contact side bearings (CCSBs) have been mandated on newer rail cars. CCSBs help reduce hunting by increasing the yaw moment between the truck and car body. The CCSBs are positioned on sides of the truck, below a wear plate positioned on the underside of the car. The cap serves as a bearing surface upon which the wear plate is in constant contact. Friction between the cap and wear plate increases the yaw moment of the truck. A resilient member beneath the cap provides the vertical force to keep the cap in constant contact with the wear plate. The side bearing must displace vertically to allow car body roll relative to the truck.
One problem with known CCSB arrangements is that the space between the upper surface of the bolster of the truck, or when used, the side bearing pedestal fixed to the bolster, and the wear plate of the car body is limited, thereby restricting the height of the CCSB to the distance between the upper surface of the bolster, or side bearing pedestal, and the car body wear plate. The limited space makes it challenging to find a resilient member with characteristics suited to withstand long term stresses encountered during operation of the rail car.